1. ELECTRONICS

2. FUNDAMENTALS OF ELECTRONICS ELECTRONICS Electronics is the branch of physics which deals with development of electron-emitting devices, there use and control of electron flow in electrical circuits. Electronics also deals with semiconductors, diode, rectifiers etc.

3. p-TYPE SUBSTANCE If a trivalent element from the IIIrd group such as Gallium (Ga) or Indium (In) is added to pure crystals of germanium (Ge) or silicone (Si), three electrons of impurity form covalent bonds with three atoms of (Ge) or (Si), while there exist a vacancy for an electron in the fourth bond. This vacant space is called Hole. This hole behaves like a positive charge and can move in the structure of substance. Such a substance is called a p-type substance.

4. n-TYPE SUBSTANCE If a pentavalent element from the Vth Group such as Antimony (Sb) is added to pure geranium (Ge) or silicone (Si), then four electrons of (Sb) will form covalent bonds with four (Ge) or (Si) atoms. The fifth electron of 'Sb' is free to move which makes (Ge) or (Si) a good conductor. This type of material is called n-type substance.

5. RECTIFIER A rectifier is a device which is used to convert alternating current (AC) into direct current (DC). PN-junction diode is used as a rectifier. RECTIFICATION The process of converting alternating current into direct current is called rectification.

6. FORWARD BIASING when n-type end of pn-junction is connected to negative terminal and p-type end with positive terminal of a (DC) supply, then the height of potential barrier reduces and provides easy flow of electric charge that is pn-junction conducts electricity. In this condition pn-junction is said to be Forward Biased.

7. REVERSE BIASING When p-type end of pn-junction is connected to the negative terminal and n-type end with positive terminal of a (DC) supply. The height of potential barrier increases to maximum and the flow of electric charge across the junction will become zero. In this condition a pn-junction diode is called Reverse Biasing.

8. DOPING Addition of an element of group IIIrd-A or Vth-A to Ge or Si crystals to convert them into semiconductor substance (p-type or n-type) is called Doping. Normally impurity is in very small quantity. There are two types of impurities that are added to geranium or silicon: DONOR IMPURITY ACCEPTOR IMPURITY

9. pn - junction diode or semiconductor diode INTRODUCTION A pn - junction diode is an electronic device formed from a p- type and an n-type substance semiconductor. A semiconductor diode has the property of one way conduction i.e. it allows electric current to flow in only one direction. FABRICATION OF pn-JUNCTION A pn-junction is fabricated by placing a small amount of indium on a plate or wafer of n -type germanium. Indium on heating at 550 o C melts and diffuses through a small part of the n-type germanium. Indium being a p-type impurity, converts the part of the n-type germanium to p-type material. Thus a junction is formed between p-type section and an n-type section of germanium. The whole apparatus is sealed in a glass tube or a metallic tube.

10. WORKING OF pn-JUNCTION DIODE As we know that a p-type substance has excess of mobile positive charge or holes and n-type substance has an excess of negative charge or electrons, the electrons from n-type and holes from p-type sections flow across the junction and combine. In this way a layer of positive charges is formed on the n-type and a layer of negative charges on p-type material. Due to induction of these layers a potential barrier is now developed across the junction and further flow of charges is prevented from one side to the other.

11. Explain the construction and working of transistor TRANSISTORS A three terminal semiconductor electronic device is called transistor. Transistors are widely used in electronic appliances such as computers, radio, audio video equipment, bio medical instrument etc. CONSTRUCTION A transistor is a three layer semiconductor which consist a very thin central layer of one type of semiconductor material sandwiched between two relatively thick layer of second type.

12. TYPES OF TRANSISTORS pnp-TRANSISTORS In this type of transistors n-type semi- condutcor piece is sandwiched between two piece of p-type semiconductor layers.

13. npn-TRANSISTORS In this type of transistors p-type semi conductor piece is sandwiched between two piece of n-type semiconductor layers.

14. ESSENTIAL PARTS OF TRANSISTORS There are three essentials parts of a transistor Base: It is the central layer denoted by b. Emitter: It is the outer layer denoted by e. Collector: It is the outer layer denoted by c.

15. WORKING Consider any one of the transistors for example a pnp-transistor. Let the two p-end are connected to two batteries as shown in the diagram. The forward bias causes the holes in the p-type emitter to flow towards the base which constituent Ie current. These holes cross into the n-type base, they try to combine with electrons but the base is lightly doped and is very thin. Therefore only few holes combine with electrons and the remaining holes cross into the collector and generates collector current Ic. In this way almost the entire emitter current flows in the collector circuit. From the above description it is clear that: Ie = Ib + Ic Thus there are two current paths through a transistor. One is the base- emitter path or input and the other is the collector-emitter path or output.